The invention relates to a current limiting device.
Current limiting devices are used in a wide variety of applications to handle fault conditions when a current surges above a safe limit. In high current applications such as power supply lines and the like, it has been known in the past to pass the current through a coil which is provided about a leg of an iron former. Another leg of the former provides the core of a superconducting coil which is activated to hold the iron former in a saturated condition. Thus, under normal conditions, the iron is saturated and so effectively the coil carrying the current sees an air core. When a fault occurs, the current rises causing a consequent increase in the magnetic field generated by the coil which opposes the field due to the superconducting coil. This causes an increase in the permeability of the iron core and this increases the voltage across the coil carrying the current which limits the current being carried.
Although this current fault limiter is effective, it is very expensive due to the need to provide the iron core and the complexities due to the need to cool the superconducting coil to liquid helium temperatures.
More recently, it has been proposed to use a superconducting switch. In this case, a length of high temperature (HTc) superconductor is placed into the circuit carrying the current. HTc materials have a critical temperature which is relatively high (typically equivalent to a liquid nitrogen temperature) and have a critical current (strictly current density) which varies inversely with an applied magnetic field. If the current carried by the superconductor exceeds the critical current then the material of the conductor makes a transition to a resistive state which acts to limit the current being carried. The critical current value at which this transition occurs can be changed by changing the applied magnetic field.
The main problem with this type of superconducting switch is that in order to reset the circuit it is necessary to break the electrical circuit completely to allow the superconductor to recover. This is due to a hysteresis effect which means that even if the current drops below the critical value, a transition back to the superconducting state will not occur because the passage of the current through the resistive material keeps the temperature high. The current has to be reduced virtually to zero for this reverse transition. Breaking the circuit is particularly undesirable since in many applications such a break cannot be tolerated.